Most think that there was only Cain and Abel born to Adam and Eve but the Bible actually teaches that by the time Cain took a wife there were many human beings, that came from Adam.

Not everything recorded in the Book of Genesis is written in an exact chronological order. The events of Genesis eleven (the Tower of Babel) happened in the middle of chapter 10, chronologically speaking –Genesis 5:4 states, "Then the days of Adam after he became the father of Seth were eight hundred years, and he had other sons and daughters." 

Both Adam and Eve driven out of the garden. Genesis 3:23-24: “therefore the LORD God sent him out of the garden of Eden to till the ground from which he was taken. So He drove out the man; and He placed cherubim at the east of the garden of Eden, and a flaming sword which turned every way, to guard the way to the tree of life.”

All their offspring were born outside the garden. Gen 5:4 "After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.”The male child born to Adam and Eve after Cain killed Abel was Seth, when Adam was 130 years old.

Gen 5:6 Seth lived one hundred and five years, and begot Enosh. Scripture  tells us "After (Adam) he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters." (Gen. 5:4). It also says of Seth "After he begot Enosh, Seth lived eight hundred and seven years, and had sons and daughters (Gen. 5:7). Daughters are not mentioned Gen.5 the phrase had sons and daughters is the key phrase but the first male born in the family is mentioned by name.

So either Cain got his wife sometime after from Seth's lineage or from Adam and Eves lineage that produced numerous children after they left the garden. We see by the record that there were a number of people by the time Cain killed his brother Abel. Genesis 4:15 And the LORD said to him, "Therefore, whoever kills Cain, vengeance shall be taken on him sevenfold." And the LORD set a mark on Cain, lest anyone finding him should kill him.”

Besides these three sons Adam and Eve had many other unnamed sons and daughters.The Bible does not give the exact time period when Cain took his wife. Depending on how much time there was between Cain being married, he either married a distant sister, or if enough time transpired, he could have married his own niece. Probably the later is true.

Marriage to a sister in the early stage of the human race was not considered wrong or unnatural. Even later on Abraham’s wife was his half-sister (20:12); also 24:4 and 28:2. God did not prohibit such marriages until the time of Moses when is was specifically stated it was against the law of God according to Lev. 18:9, 18:11, 20:17, and Deut. 27:22.

Genesis 4:16-17: “Then Cain went out from the presence of the LORD and dwelt in the land of Nod on the east of Eden. And Cain knew his wife, and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son-- Enoch.”

We can see by how it is written it is a summary of a whole period of time. He married, his wife conceived and they even built a city; obviously with other people in the area he lived (Nod).